Polyamides are condensation products containing recurring amide groups, generally prepared by the condensation of a diamine and a dibasic acid or their equivalents, or the polymerization of bifunctional monomers. Polyamides are frequently referred to as nylons, and include, for example nylon 4, nylon 6, nylon 6,6, nylon 6,9, nylon 6, 12, nylon 7, nylon 8, nylon 9, nylon 11, nylon 12, etc. Polyamides generally exhibit good solvent resistance, hydrolytic stability, abrasion resistance and mechanical strength due to their highly crystalline structure. They, however, do exhibit poor dimensional stability in humid environments due to water absorption, as well as poor creep performance at typical use temperatures.
Polycarbonates are well-known commercially available resinous materials having a variety of applications. They are typically prepared by the reaction of dihydroxy compounds and a carbonate precursor, such as phosgene. Polycarbonates are high temperature, high performance thermoplastic engineering polymers with a good combination of thermal and mechanical properties, especially when the polymer is prepared from one or more aromatic diols. However, polycarbonates do exhibit poor solvent resistance, particularly when exposed to humid environments for prolonged times and/or at elevated temperatures.
Polycarbonate/polyamide blends might be expected to have a useful balance of properties. Such blends could possess improved creep resistance and higher temperature resistance due to the polycarbonate component, and improved solvent resistance as a result of the polyamide component. Unfortunately, attempts to modify the properties of polycarbonate resins by blending with polyamides are quite often unsatisfactory because of their incompatibility. Simple blends of polycarbonate and polyamide resins are deficient in mechanical properties such as elongation and impact strength, thereby suggesting low interfacial adhesion.
Most combinations of polymers are not compatible, although a number of notable exceptions are known. Generally, polymers adhere poorly to one another resulting in interfaces between the component domains which provide natural sites for mechanical failures due to flaws and crack propagation. Because of this, the polymers are said to be "incompatible". Occasionally, such polymer blends may be compatibilized by adding a third component, referred to as a compatibilizing agent, which possesses a dual solubility characteristic for the two polymers. The compatibilizing agent generally locates at the interface between the polymers and greatly improves interfacial adhesion, and thereby increases the stability to gross phase separation.
U.S. Pat. No. 4,111,895 discloses the use of a block copolymer, such as styrene-butadiene-styrene, as a compatibilizing agent between polycarbonate and a host of dissimilar thermoplastics, including polyamide. U.S. Pat. No. 4,317,891 discloses a blend of polycarbonate, polyamide, and a conjugated diene rubber copolymer. The copolymer does not, however, act as a compatibilizing agent, and the amount of polyamide is limited to 20 weight percent due to incompatibility with the polycarbonate at higher concentrations. Finally, U.S. Pat. No. 3,658,752 discloses a blend of an elastomer and a polyamide, and additionally contains as a third component a filler which acts as a compatibilizing agent.